Gas turbine power plants are used as the primary propulsive power source for aircraft, in the forms of jet engines and turboprop engines, as auxiliary power sources for driving air compressors, hydraulic pumps, etc. on aircraft, and as stationary power supplies such as backup electrical generators for hospitals and the like. The same basic power generation principles apply for all of these types of gas turbine power plants. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge upon turbine blades mounted on a turbine disk or wheel that is free to rotate.
The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use this power to draw more air into the engine and then high velocity combustion gas is passed out the aft end of the gas turbine, creating forward thrust. Other engines use this power to turn a propeller or an electric generator.
The turbine blades and vanes lie at the heart of the power plant, and it is well established that in most cases, they are one of the limiting factors in achieving improved power plant efficiency. In particular, because they are subjected to high heat and stress loadings as they are rotated and impacted by the hot gas, there is a continuing effort to identify improvements to the construction and/or design of turbine blades to achieve ever higher performance.
Much research and engineering has been directed to the problem of improved turbine blade materials. The earliest turbine blades were made of simple cast alloys having relatively low maximum operating temperatures. The alloy materials have been significantly improved over a period of years, resulting in various types of nickel-based and cobalt-based superalloys that are in use today.
As the alloy materials were improved, the metallurgical microstructure of the turbine blades was also improved. First, the polycrystalline grain structures were modified by a wide variety of treatments to optimize their performance. Directionally solidified or oriented polycrystalline blades were then developed, having elongated grains with deformation-resistant orientations parallel to the radial axis of the blade in order to best resist the centrifugal stresses. Each of these advancements led to improved performance of the blades. Polycrystalline and oriented polycrystalline blades are widely used in most commercial and many military aircraft engines today.
It has been proposed to improve polycrystalline blades by including reinforcing ceramic fibers or the like in the structure but such approaches have not met with success primarily because of the problems in adequately bonding such differing materials so that operating stresses are evenly distributed.
More recently, single crystal turbine blades have been introduced as a result of the development of practical techniques to cast them in large quantities. These turbine blades have the advantage of eliminating grain boundaries entirely, which are one of the important causes of creep deformation and failure of the airfoil. The elimination of grain boundaries allows the chemical composition of the single crystal blade to be adjusted to achieve improved creep and high-cycle fatigue performance at the highest engine operating temperatures. Single crystal turbine blades are now used in high performance military aircraft and may eventually be introduced into commercial applications.
While the single crystal turbine blades have provided improved overall airfoil performance as compared with polycrystalline blades, they still exhibit problem areas. In many applications, the highly loaded attachment area is subject to low cycle fatigue failures. As a result, there is a continuing need to provide yet further improvements to achieve higher operating temperatures and lengthened operating lives in the blades used in high performance gas turbine engines.
It is therefore an object of the present invention to provide a novel turbine blade, and method of making same, which has an increased operating life.
Another object of the invention is to provide a single crystal turbine blade having a reduced susceptibility to failure in its attachment area.
A further object of the invention is to provide a composite structure in at least a portion of the attachment section of a single crystal turbine blade to retard creep and/or crack growth in said portion.